The present invention relates to read post compensation, also known as bit shift compensation, for the reading of digital data from magnetic media, such as floppy and hard disk and tape drives.
In the recording of digital data on magnetic media, the bits are recorded as flux transitions in the magnetic medium. As bits are recorded close together, as in high bit density recording (especially on the inner tracks of a disk), the flux transitions representing the bits are located very close together on the magnetic medium. In recording, the poles in the medium are aligned so that the north poles are adjacent to one another while the south poles are also adjacent to one another. It is well known from elementary physics that like poles repel. Thus, these like poles tend to cancel one another. Since the recording of data involves changing the spacing between flux reversals, some reversals are closer to one another than others are. Thus, the amount of the cancellation varies within the flux pattern. The closer reversals are cancelled more than the ones further apart. This effect tends to spread the closer reversals further apart while the more widely spaced reversals tend to be bunched more closely together. Thus, the flux transitions do not occur exactly on their nominal locations. When the flux transitions are later read from the magnetic medium, they appear to be displaced in time. This displacement is known as bit shift.
In many cases, the controller for the drive for the magnetic medium is called upon to perform write precompensation to compensate for the bit shift effect. The close flux transitions are called upon to be written closer together while the ones spaced further apart are made even further apart to compensate for the effect described above. This technique is only approximate, however, as the drive controller cannot know exactly how the drive is going to displace the bits on recording.
There is thus a need for a circuit to compensate exactly for bit shift so that exactly the proper amount of compensation can be applied for the drive. This will improve the reliability with which the drive reads the data that were recorded. This could be done on the write side of the drive as well as the read side. However, there usually exists one or more one shot timing circuits in the read chain of these drives that allows for easier implementation of a compensating circuit than in the write side, which usually has no one shot timing circuits.
It is an object of the invention to provide a read post compensator circuit to dynamically vary the period between successive output bits in direct relation to the time interval between successive read bits to compensate exactly for bit shift.